Method for reducing crazing in a plastics material

ABSTRACT

A method for reducing crazing in a plastics material characterized in that it comprises the steps of:  
     (1) cleaning the surface of the material; and  
     (2) exposing the cleaned surface to plasma of a monomer vapor so as to produce a substantially non-oxidizing plasma polymer coating on the surface.

[0001] The present invention relates to a method for reducing crazing ina plastics material, in particular a transparent plastics material.

[0002] When transparent plastics materials are used for windows,roofing, signs or light fittings maintenance of their original opticalclarity is important. Unfortunately, under the influence ofenvironmental factors such as light, heat and moisture many plasticsmaterials suffer from crazing. Crazing is a phenomenon where microvoidsform in the body of the materials. These microvoids may not cause asignificant deterioration in mechanical strength of the article, butthey do reflect/refract light and decrease the clarity of the article.Ultimately, crazing decreases the strength of the article and can leadto failure.

[0003] As crazing is a visually obvious deterioration in the material,it gives an impression of poor quality or lack of maintenance which isparticularly objectionable in applications where visual clarity isdesired. Such applications include the windows of transport vehicles,roofing sheets, light fittings or signs. Many signs are made oftransparent materials with the graphic material applied to the undersideto be viewed through the material. Crazing in vehicle windows interfereswith the vision of the occupants decreasing their enjoyment of thejourney and may even pose a real safety hazard. Crazing is particularlyobjectionable in aircraft windows and frequently causes the replacement,at a great expense, of windows which are otherwise sound andserviceable. As a consequence of the crazing problem, the use of glasswindows is being considered for aircraft despite the weight penalty thatthis would impose.

[0004] The cause of crazing is unknown and may be manifold. It isthought that one cause is the diffusion of small molecules such as wateror surfactants into the material which decreases the attractive forcesbetween polymer chains and allows movement of molecules under internalor external stress thus forming microvoids.

[0005] According to the present invention there is provided a method forreducing crazing in a plastics material which comprises the steps of:

[0006] (1) cleaning the surface of the material; and

[0007] (2) exposing the cleaned surface to plasma of a monomer vapour soas to produce a substantially non-oxidising plasma polymer coating onthe surface.

[0008] The method of the present invention may be used to reduce crazingin a wide variety of plastics materials, such as, for example, acrylics,styrenes, polycarbonates, polyesters or polyurethanes. The plasticsmaterial may be an article which is preferably in the form of a laminateor sheet. The method will have particular value when applied totransparent plastics material where visual clarity is important.Examples of transparent materials include acrylic or polycarbonatesheets as used for the windows of transport vehicles such as aircraft,boats, trains and motor vehicles, signs or for architectural uses suchas in roofing, glazing sheets and light fittings.

[0009] The material may be cleaned in step (1) by any method whichleaves the surface substantially free of any contamination capable ofinterfering with the adhesion of the plasma polymer coating. A preferredmethod of cleaning the surface is to subject the material to a lowpressure plasma of an inert gas such as argon, neon, or nitrogen.Another preferred method of cleaning the surface involves subjecting thematerial to a low pressure plasma of an oxidising gas such as air oroxygen. Water vapour is also a suitable oxidising gas for this purpose.These cleaning methods may be advantageously carried out in the sameapparatus which is used in step (2) of the method.

[0010] The monomer used in step (2) may be any saturated or unsaturatedorganic compound capable of producing a coating of a substantiallynon-oxidising polymer containing organic groups.

[0011] Suitable saturated monomers include siloxanes, fluorinatedcompounds, lower hydrocarbons, lower alcohols, lower alkylamines andmixtures thereof. The term “lower” as used herein refers to monomerscontaining 1 to 12 carbon atoms.

[0012] Suitable unsaturated monomers include acrylic esters, methacrylicesters, vinyl esters, vinyl aromatics, unsaturated or polyunsaturatedhydrocarbons and mixtures thereof. Examples of these monomers includemethyl acrylate, ethyl acrylate, butyl acrylate, 2-ethylhexyl acrylate,methyl methacrylate, ethyl methacrylate, n-butyl methacrylate,2-ethylhexyl methacrylate, vinyl acetate, styrene,p-chloromethylstyrene, 2-vinylpyridine, 4-vinylpyridine,N-vinylpyrrolidone, vinyl halides of the formula CH₂═CHX wherein X in Clor F, vinylidene halides of the formula CH₂═CX₂ wherein X isindependently Cl or F, vinyl ethers of the formula CH₂═CHOR wherein R isalkyl, and allyl derivatives such as allyl ethers, allyl carbonates ordiallyl carbonates.

[0013] Plasma polymers from some of these monomer classes typicallyundergo extensive oxidation on aging (Gegennbach et al, J Polymer Sci,Part A Polymer Chemistry, 32, 1399-1414 (1994); Gegennbach et al,Surface Interface Analysis, in press 1996). In those cases it isnecessary to carefully adjust the plasma deposition parameters untilminimal oxidation following ageing in the air is obtained. Whilesubstantial oxidation can occur in plasma polymers without affectingtheir structural integrity, minimal oxidation lessens the danger ofadverse changes to the surface or mechanical properties of a plasmapolymer as it ages. As used herein, the term “substantiallynon-oxidising polymer” refers to materials which show such minimaloxidation.

[0014] It has been found that for windows made from acrylic polymerssuch as those used in aircraft, the substantially non-oxidising polymercoating is preferably hydrophobic. Siloxanes or perfluorinated compoundsare particularly suitable monomers for producing hydrophobic coatingsprovided that the resulting polymer contains some organic groups.Examples of such monomers include hexamethyldisiloxane,vinyltrimethoxysilane, perfluorocyclohexane and tetrafluoroethylene.

[0015] For plastics materials where the crazing is caused by exposure tohydrophobic molecules such as petroleum products, a hydrophilic coatingmay be more suitable in which case monomers such as alcohols oralkylamines may be used. Preferred examples of such monomers includemethanol, ethanol and the various isomers of propanol or butanol.

[0016] The plasma polymer coatings produced by the method of thisinvention are usually highly crosslinked and hence stable. They may alsobe abrasion resistant.

[0017] Many of the materials to which the method of the presentinvention can be applied are subject to varying stresses in service andmove or flex slightly as a consequence. Accordingly, there is a need tomatch the mechanical compliance of the coating with that of thematerial. The present invention achieves this by ensuring that theplasma polymer coating applied in step (2) is thin and adheres well tothe material so that it moves with the material without itself crackingor crazing. It is preferred that the plasma polymer coating has athickness of about 2 to about 500 nm, more preferably about 5 to about50 nm.

[0018] The prior art methods produce thicker coatings which are unableto follow the movement of the material and crack and/or delaminate.

[0019] The method of the present invention may be carried out in anysuitable apparatus for performing plasma polymerisation such as thatdescribed in AU 654131. AU 654131 describes a process for plasma coatingpolymeric materials in a vapour of an amide monomer so as to provide acoating suitable for the growth of cells on biomedical implants to beadministered into the human body. Preferably, low pressure plasmapolymerisation is employed in which the pressure is about 0.5 to about1.0 torr.

[0020] The present invention also provides a craze resistant articlecomprising a plastics material having a thin coating of a substantiallynon-oxidizing plasma polymer containing organic groups.

[0021] This invention is further explained and illustrated in thefollowing non-limiting examples.

EXAMPLE 1 Coating of Acrylic Plastic Sheet

[0022] Test strips of 35 cm×3 cm were cut from a 3 mm thick acrylicsheet. Each strip was cleaned by wiping with toluene-soaked, lint-freetissues and repeated twice further with fresh tissues. The finalpreparation of the surface was achieved by treatment in an air plasmaunder the following conditions:

[0023] 0.55 Torr pressure of air

[0024] 225 kHz frequency

[0025] 10 Watt load power

[0026] 60 secs duration.

[0027] The coating was applied to the air plasma cleaned sample byexposure to a plasma in hexamethyl disiloxane vapour under the followingconditions:

[0028] 0.11 Torr pressure

[0029] 225 kHz frequency

[0030] 50 Watt load power

[0031] 240 secs duration.

[0032] A strong adherent coating of plasma polymer was formed.

EXAMPLE 2 Evaluation of Effect of Coatings on Craze Resistance

[0033] Test strips prepared according to Example 1 were tested forsusceptibility to crazing using a modification of the cantilever testmethod of Burchill, Mathys and Stacewicz (J. Materials Science 22,483-487 (1987)) which is a modification of the standard test methodANSI/ASTM F484-77 “Stress crazing of acrylic plastics in contact withliquid or semi-liquid compounds”. The samples were 35 cm long. A weightof 1 kg was suspended from the unsupported end of the test strip. Theload was applied for 10 mins before placing the test fluid(iso-propanol) on the tensile surface which was kept wet untilexamination for crazing (at least a further 20 mins).

[0034] Uncoated control strips cut from the same sheet crazed within 20mins. However, the strips prepared in Example 1 did not craze after 6hrs when the test was halted.

EXAMPLE 3 Coating of Commercial Acrylic Sheet With n-Heptylamine PolymerAfter Air Plasma Cleaning

[0035] Test strips of 35 cm×3 cm were cut from 3 mm thick commercialcast acrylic sheet. Each strip was cleaned by wiping withtoluene-soaked, lint-free tissues, and wiping repeated twice more withfresh tissues. Final preparation of the surface was achieved bytreatment in an air plasma under the following conditions:

[0036] 0.50 Torr pressure

[0037] 200 kHz frequency

[0038] 10 Watt load power

[0039] 60 second duration.

[0040] The coating was applied to the plasma-cleaned sample by exposureto a plasma in n-heptylamine vapour under the following conditions:

[0041] 0.40 Torr pressure

[0042] 200 kHz frequency

[0043] 10 Watt load power

[0044] 180 second duration.

[0045] A coating of plasma polymer ca 120 nm thick formed.

EXAMPLE 4 Coating of Commercial Acrylic Sheet With n-Heptylamine PolymerAfter Argon Plasma Cleaning

[0046] Test strips of 35 cm×3 cm were cut from 3 mm thick commercialcast acrylic sheet. Each strip was cleaned by wiping withtoluene-soaked, lint-free tissues, and wiping repeated twice more withfresh tissues. Final preparation of the surface was achieved bytreatment in an argon plasma under the following conditions:

[0047] 0.50 Torr pressure

[0048] 200 kHz frequency

[0049] 10 Watt load power

[0050] 60 second duration.

[0051] The coating was applied to the plasma-cleaned sample by exposureto a plasma in n-heptylamine vapour under the following conditions:

[0052] 0.40 Torr pressure

[0053] 200 kHz frequency

[0054] 20 Watt load power

[0055] 180 second duration.

[0056] A coating of plasma polymer ca 110 nm thick formed.

EXAMPLE 5 Coating of Commercial Acrylic Sheet With n-Hexane PolymerAfter Air Plasma Cleaning

[0057] Test strips of 35 cm×3 cm were cut from 3 mm thick commercialcast acrylic sheet. Each strip was cleaned by wiping withtoluene-soaked, lint-free tissues, and wiping repeated twice more withfresh tissues. Final preparation of the surface was achieved bytreatment in an air plasma under the following conditions:

[0058] 0.50 Torr pressure

[0059] 200 kHz frequency

[0060] 10 Watt load power

[0061] 60 second duration.

[0062] The coating was applied to the plasma-cleaned sample by exposureto a plasma in n-hexane vapour under the following conditions:

[0063] 0.11 Torr pressure

[0064] 200 kHz frequency

[0065] 20 Watt load power

[0066] 120 second duration.

[0067] A coating of plasma polymer ca 140 nm thick formed.

EXAMPLE 6 Coating of Commercial Acrylic Sheet With n-Hexane PolymerAfter Argon Plasma Cleaning

[0068] Test strips of 35 cm×3 cm were cut from 3 mm thick commercialcast acrylic sheet. Each strip was cleaned by wiping withtoluene-soaked, lint-free tissues, and wiping repeated twice more withfresh tissues. Final preparation of the surface was achieved bytreatment in an argon plasma under the following conditions:

[0069] 0.50 Torr pressure

[0070] 200 kHz frequency

[0071] 10 Watt load power

[0072] 60 second duration.

[0073] The coating was applied to the plasma-cleaned sample by exposureto a plasma in n-hexane vapour under the following conditions:

[0074] 0.40 Torr pressure

[0075] 200 kHz frequency

[0076] 20 Watt load power

[0077] 120 second duration.

[0078] A coating of plasma polymer ca 130 nm thick formed.

EXAMPLE 7 Coating of Commercial Acrylic Sheet With Methanol PolymerAfter Air Plasma Cleaning

[0079] Test strips of 35 cm×3 cm were cut from 3 mm thick commercialcast acrylic sheet. Each strip was cleaned by wiping withtoluene-soaked, lint-free tissues, and wiping repeated twice more withfresh tissues. Final preparation of the surface was achieved bytreatment in an air plasma under the following conditions:

[0080] 0.50 Torr pressure

[0081] 200 kHz frequency

[0082] 10 Watt load power

[0083] 60 second duration.

[0084] The coating was applied to the plasma-cleaned sample by exposureto a plasma in methanol vapour under the following conditions:

[0085] 0.60 Torr pressure

[0086] 200 kHz frequency

[0087] 20 Watt load power

[0088] 600 second duration.

[0089] A coating of plasma polymer ca 51 nm thick formed.

EXAMPLE 8 Coating of Commercial Acrylic Sheet WithPerfluorodimethylcyclohexane Polymer After Air Plasma Cleaning

[0090] Test strips of 35 cm×3 cm were cut from 3 mm thick commercialcast acrylic sheet. Each strip was cleaned by wiping withtoluene-soaked, lint-free tissues, and wiping repeated twice more withfresh tissues. Final preparation of the surface was achieved bytreatment in an air plasma under the following conditions:

[0091] 0.50 Torr pressure

[0092] 200 kHz frequency

[0093] 10 Watt load power

[0094] 60 second duration.

[0095] The coating was applied to the plasma-cleaned sample by exposureto a plasma in perfluorodimethylcyclohexane vapour under the followingconditions:

[0096] 0.2 Torr pressure

[0097] 200 kHz frequency

[0098] 5 Watt load power

[0099] 180 second duration.

[0100] A coating of plasma polymer ca 120 nm thick formed.

EXAMPLE 9 Coating of Commercial Acrylic Sheet With Methyl MethacrylatePolymer After Air Plasma Cleaning

[0101] Test strips of 35 cm×3 cm were cut from 3 mm thick commercialcast acrylic sheet. Each strip was cleaned by wiping withtoluene-soaked, lint-free tissues, and wiping repeated twice more withfresh tissues. Final preparation of the surface was achieved bytreatment in an air plasma under the following conditions:

[0102] 0.50 Torr pressure

[0103] 200 kHz frequency

[0104] 10 Watt load power

[0105] 60 second duration.

[0106] The coating was applied to the plasma-cleaned sample by exposureto a plasma in methyl methacrylate vapour under the followingconditions:

[0107] 0.5 Torr pressure

[0108] 200 kHz frequency

[0109] 10 Watt load power

[0110] 60 second duration.

[0111] A coating of plasma polymer ca 210 nm thick formed.

EXAMPLE 10 Coating of Commercial Acrylic Sheet With n-Butyl MethacrylatePolymer After Air Plasma Cleaning

[0112] Test strips of 35 cm×3 cm were cut from 3 mm thick commercialcast acrylic sheet. Each strip was cleaned by wiping withtoluene-soaked, lint-free tissues, and wiping repeated twice more withfresh tissues. Final preparation of the surface was achieved bytreatment in an air plasma under the following conditions:

[0113] 0.50 Torr pressure

[0114] 200 kHz frequency

[0115] 10 Watt load power

[0116] 60 second duration.

[0117] The coating was applied to the plasma-cleaned sample by exposureto a plasma in n-butyl methacrylate vapour under the followingconditions:

[0118] 0.5 Torr pressure

[0119] 200 kHz frequency

[0120] 5 Watt load power

[0121] 120 second duration.

[0122] A coating of plasma polymer ca 125 nm thick formed.

EXAMPLE 11 Evaluation of Effect of Coatings on Craze Resistance to PolarMaterials

[0123] Test strips prepared according to Examples 3-10 were tested forsusceptibility to crazing using a modification of the cantilever testmethod of Example 2. The samples were 35 cm long and a weight of 1 kgwas suspended from the unsupported end of the test strip. The load wasapplied and the test fluid (isopropanol) applied immediately to thetensile surface which was kept wet and under observation until crazingoccurred.

[0124] Although all test strips eventually crazed, all treated stripslasted at least ten times longer than uncoated control strips.

[0125] These results demonstrate that the process of the inventionenhances the craze resistance of commercial acrylic sheet such as thatused for glazing or signs as well as the stretched acrylic sheet usedfor aircraft windows.

[0126] Throughout this specification, unless the context requiresotherwise, the word “comprise”, or variations such as “comprises” or“comprising”, will be understood to imply the inclusion of a statedinteger or group of integers but not the exclusion of any other integeror group of integers.

1. A method for reducing crazing in a plastics material characterised in that it comprises the steps of: (1) cleaning the surface of the material; and (2) exposing the cleaned surface to plasma of a monomer vapour so as to produce a substantially non-oxidising plasma polymer coating on the surface.
 2. A method as claimed in claim 1, characterised in that the monomer in step (2) is a saturated or unsaturated organic compound capable of producing a coating of a substantially non-oxidising polymer containing organic groups.
 3. A method as claimed in claim 2, characterised in that the monomer is a saturated monomer selected from the group consisting of siloxanes, fluorinated compounds, lower hydrocarbons, lower alcohols, lower alkylamines and mixtures thereof.
 4. A method as claimed in claim 2, characterised in that the monomer is an unsaturated monomer selected from the group consisting of acrylic esters, methacrylic esters, vinyl esters, vinyl aromatics, unsaturated or polyunsaturated hydrocarbons and mixtures thereof.
 5. A method as claimed in claim 4, characterised in that the monomer is selected from the group consisting of methyl acrylate, ethyl acrylate, butyl acrylate, 2-ethylhexyl acrylate, methyl methacrylate, ethyl methacrylate, n-butyl methacrylate, 2-ethylhexyl methacrylate, vinyl acetate, styrene, p-chloromethylstyrene, 2-vinylpyridine, 4-vinylpyridine, N-vinylpyrrolidone, vinyl halides of the formula CH₂═CHX wherein X in Cl or F, vinylidene halides of the formula CH₂═CX₂ wherein X is independently Cl or F, vinyl ethers of the formula CH₂═CHOR wherein R is alkyl, and allyl derivatives such as allyl ethers, allyl carbonates or diallyl carbonates.
 6. A method as claimed in claim 1 or claim 2, characterised in that the plastics material is an acrylic polymer and the polymer coating is a siloxane or perfluorinated compound.
 7. A method as claimed in claim 6, characterised in that the polymer is produced from a monomer selected from the group consisting of hexamethyldisiloxane, vinyltrimethoxysilane, perfluorocyclohexane and tetrafluoroethylene.
 8. A method as claimed in claim 1 or claim 2, characterised in that the plastics material is to be used in an environment where it is exposed to hydrophobic molecules and the polymer coating is a hydrophilic coating.
 9. A method as claimed in claim 8, characterised in that the polymer is produced from a monomer produced from an alcohol or alkylamine.
 10. A method as claimed in claim 9, characterised in that the polymer is produced from an alcohol selected from the group consisting of methanol, ethanol and the various isomers of propanol or butanol. 